To ensure uninterrupted operation of a computer system dependent upon the receipt of power from an external AC power source, and thereby prevent the loss of data and equipment damage, uninterruptable power source (UPS) systems have been developed to supply the electrical power needs of computer systems during disruptions in primary AC power sources.
The typical UPS includes an inverter for converting a source of stored energy into alternating current power having the same frequency and voltage as the primary AC power source for the computer. An on-line UPS continuously provides power from the inverter regardless of the state of the primary AC power. On-line UPS systems are normally provided for large scale computers. Smaller, less-expensive off-line, or standby, UPS systems which provide power directly from the primary AC power source until an AC power failure is detected are better suited for small computer system applications. Upon detection of a primary AC power failure, the standby UPS system operates to provide power from the stored energy source through the inverter to the computer system.
One such UPS system for supplying power to a group of small computers is disclosed in U.S. Pat. No. 4,611,289. The system includes a standby UPS and a bank of batteries for supplying backup power to the computers when utility power is interrupted. The system further includes a microprocessor which monitors the utility power state and the remaining energy level stored in the back-up batteries. The microprocessor provides to the computer a first signal indicating the loss of primary utility power and a second signal indicating restoration of primary utility power. The first signal is designed to instruct non-critical task computers to perform an orderly shut down. However, the initiation of shut down of critical task computers is delayed until a low energy condition in the back-up batteries is detected.
The shut down routine may include the steps of halting program execution and storing the contents of volatile memory in non-volatile memory, closing open files, logging users off the system, and removing power from the system. The shut down routine may also include procedures for resetting or restarting the computer system in the event primary AC power is restored to the system prior to the completion of all shut down procedures.
Although shut down routines are well known in the art, improvements in such routines are desirable. Improvements may include reducing interference with normal operations and inconvenience to users, delaying of initiation of critical and non-critical program execution curtailment until essential, increasing opportunity for system and application recovery in the event primary AC power is restored, and allowing the user to define application shut down procedures and select ride-through and shut-down time periods.